In many applications involving the processing of electronic signals it is desirable to initially filter the signals to remove unwanted frequencies and spurious noise and to select a small, predetermined frequency range for further processing. Such a filtering operation is typically performed with a bandpass filter.
Many prior filter circuits have been designed in an attempt to produce a filter circuit which has dual characteristics of a high amount of signal attenuation for signals with frequencies outside the predetermined frequency range (designated as the "passband") and a low amount of signal attenuation for signals having frequencies inside the passband.
The filtering problem is further complicated because many electronic circuits process signals which have frequencies in several distinct ranges. One such electronic circuit is a modem, a device that allows electronic computers to communicate via ordinary telephone lines. The modem converts electronic digital signals produced by one computer into audio-frequency signals which are then transmitted over a telephone line. A corresponding modem at the other end of the telephone line receives the audio-frequency signals and reconverts them into digital signals which are then used by another computer.
In a typical prior art modem communication system, two different sets (four frequencies total) are used to provide full duplex operation. The audio-frequencies generated and received by each modem may be selected from either set depending on whether the modem originated the telephone connection or merely answered a telephone call placed by another modem. Typically, a modem must be able to receive a small band of audio-frequencies with an approximate center frequency of 1170 Hertz if it is in the "answer" mode and a center frequency of 2100 Hertz if it is in the "originating" mode. The width of the passband is typically 400 Hertz in both cases.
In order to filter signals in two separate ranges with widely spaced center frequencies, it is necessary to design two bandpass filters with substantially different characteristics. Therefore, a typical prior art solution to this problem has been to use two separate filter circuits which have many duplicated components. This solution, although straightforward, is not totally satisfactory when applied to a modem in which only one filter circuit is in use at any given time (depending on whether the modem is in the "answer" or "originate" mode). Obviously this prior art approach results in duplication of components and increased costs.
To avoid duplication of parts, other prior art attempts have been made to design a single bandpass filter having two separate passbands by utilizing an active filter circuit with multiple feedback loops and interchanging components in the filter networks to change the center frequency of the filter. These attempts have encountered problems, however, because the signal gain of such a switchable filter usually changes when the values of the components in the filter networks are changed.
It is therefore an object of the invention to provide a switchable bandpass filter having passbands with two separate center frequencies which utilizes a minimum number of components.
It is a further object of the invention to provide a switchable bandpass filter in which the overall signal gain remains constant even though the center frequency changes.
It is a still further object of the invention to produce increased sensitivity and selectivity of the bandpass filter circuitry.